The invention is related to the field of threaded couplings such as used to join segments of drill pipe and the like. More specifically, the invention is related to types of threaded connectors that can withstand very high differential fluid pressures, both internal to and external to coupled pipe segments.
Two-step threaded connections are known in the art for providing high capacity to resist internal and external differential pressures in a pipe assembled from threadedly coupled segments. The threaded connections form metal-to-metal seals which are capable of holding the differential pressures under a wide range of axial stresses and bending stresses on the assembled pipe. Examples of two-step threaded connections are shown in U.S. Pat. No. 4,662,659 issued to Blose et al, and in U.S. Pat. No. 4,688,832 issued to Ortloff et al.
The two-step threaded connectors in both the ""832 and ""659 patents include a positive stop torque shoulder which limits the axial movement between female and male portions of the threaded coupling as the coupling is assembled and tightened to the proper torque. In some cases, the assembled pipe segments are subjected to very high axial compressive loading. It is desirable to have a threaded connection which can withstand even higher axial compressive loading than the connections known in the art so that the reliability of the connector can be increased.
One aspect of the invention is a threaded connector which includes a pin member having a large thread step and a small thread step. The pin member has a first sealing surface between the steps. Threads on the steps of the pin member each have load flanks and stab flanks. The connector includes a box member having a large thread step and a small thread step corresponding to the thread steps on the pin member. The box member has a second sealing surface corresponding to the first sealing surface. Threads on the steps of the box member have load flanks and stab flanks corresponding to the threads on the pin member. The threads on the large thread step have a first clearance between corresponding stab flanks on the pin and box members when the pin and box members are assembled and the corresponding load flanks are engaged. The threads on the small step have a second clearance between corresponding stab flanks when the pin and box members are assembled. In one embodiment, the first clearance is smaller than the second clearance so that axial movement between the pin and box is limited. This can increase the axial compression capability of the connector.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.